Trading opened in New York at 14.30BST for the first time since the relevation and, after just 10 minutes, United posted a 4.7 per cent drop in share price from $18.77 [£12.08] to $17.88 [£11.51].

After an hour, United's price appeared to stabilise after it had risen back up to $18.41 [£11.84] but fluctuations have continued ever since.

United, owned by the Florida-based Glazer family, were floated on the the New York Stock Exchange for the first time in August 2012, while last Thursday, the club posted a record third-quarter turnover of £91.7 million.

With Sir Alex to retire at the end of the season and take up a director and ambassador position, Goal.com exclusively revealed that Everton manager David Moyes will replace the 71-year-old at Old Trafford and could be confirmed within the next 24 hours.

In his statement, Sir Alex believed that "now is the right time" for him to step down but insisted that he is leaving the side "in the strongest possible shape".

United chief executive David Gill confirmed that Sir Alex, along with Sir Bobby Charlton, are involved in the appointment of the new manger.

"The qualities [needed] are the ones that have been inherent in Manchester United for many years," Gill told MUTV.

"If you look at what has happened with the two most successful eras – Sir Matt Busby and Alex – they are managers who got involved in the whole aspect of the club, whether it be from the youth team up to the first team.

"All aspects of it, plus that degree of loyalty and the understanding of the football club that it's not just what happens on the first-team pitch, is crucial to the success of Manchester United. Those are the sort of things we’ll be looking at.

"Clearly he has to have the requisite football experience, both in terms of domestic and European experience, so I think it's a small pool but we'll move forward."